Electro-optic devices such as light sources and detectors are becoming of increased technological importance in communications, range finding and target designation applications. In applications such as these the device housing must provide optical and electrical access to the electro-optic device as well as a hermetic seal against the ambient. The profile of the device housing is also oftentimes important in certain applications.
Electro-optic device housings typically are similar to that described by Collins et al, U.S. Pat. No. 3,805,347, issued Apr. 23, 1974, which shows a cap, which contains a window, fastened to a header at a projecting flange. Techniques used for fastening the cap to the header at the projecting flanges include soldering, electrical resistance welding, as described for example in the RWMA Manual published by the Resistance Welders Manufacturer's Association, and cold pressure welding as described in U.S. Pat. No. 3,309,175, issued June 19, 1962, to Dixon.
To obtain a hermetically sealed package a glass to metal seal of the window to the housing cap and standard resistance welding at the projecting flanges can be used when the window is separated from the weld flange by a suitable distance such that the window is not heated to a significant degree during the welding process. Significant heating of the glass to metal sealing joint can lead to failure of the seal or to cracking of the window. If the window-to-flange spacing is small then proper heat sinking is required in order to protect the glass to metal seal.
For some applications an electro-optic device housing without projecting flanges is required. The cap and header must then be butt welded to one another to form the required hermetic seal. In this case the welding electrode must contact the wall of the cap on its perimeter. This electrode must provide good electrical and thermal contact to the wall to minimize electrical and thermal contact resistance. Electrical contact resistance must be minimized so that the applied electrical power is dissipated at the point of welding. Thermal contact resistance must be minimized so that the electrode can provide a heat sink for heat generated at the weld joint which diffuses into the wall of the cap.
Thus, it would be desirable to have a method and apparatus for butt welding a heat sensitive cap to a header which minimizes the electrical and thermal contact resistance between the cap and the header.